Recently, with developments on digitalization of electric devices and higher-speed personal computers, capacitors, downsized but having large capacitance and low impedance in a high-frequency region, are demanded, and there have been proposed solid electrolytic capacitors using electroconductive polymer having electron conductivity as solid electrolyte.
In a production method of a solid electrolytic capacitor (9), generally, a solid electrolytic capacitor element is produced by forming a dielectric oxide film layer (2) on an anode substrate (1) consisting of a metal foil or a thin sheet having a large specific surface area increased by etching treatment as shown in FIG. 1, forming thereon a solid semiconductor layer (3) (hereinafter referred to as solid electrolyte) as a counter electrode, and further forming a conductor layer (4) of electroconductive paste or the like. Generally, a masking layer (5) is further provided thereto, electrode leads (6, 7) are added as appropriate and the whole is to be encapsulated by resin (8) to become a capacitor.
A silver paste is mainly used for the above-mentioned conductor layer (4). Since silver readily migrates at a high temperature, generally, a silver paste layer (4b) comprising silver particles is formed on a carbon paste layer (4a) comprising carbon particles (see FIG. 2). These layers can be formed by dipping the anode substrate sequentially in liquid containing the component of each layer, respectively. Since the substrate of a capacitor element is generally in a shape of a thin plate, the thickness of the formed silver paste layer becomes different in the face part and in the side (edge) part of the substrate as in FIG. 2. Specifically, the thickness of the layer tends to be reduced on the side (edge) part, and as a result, equivalent series resistance (ESR) is likely to be high, which is undesirable. Though the thickness of the layer on the side (edge) part can be increased by applying a thicker layer to the whole of the substrate, in this case, the layer becomes thicker on the face part as well, and therefore the thickness per element increases, which lowers the capacitance per unit volume of a laminated capacitor produced by laminating the elements. Accordingly, a solid electrolytic capacitor produced by laminating silver paste is demanded in which the layer thickness on the face part is suppressed while that on the side (edge) part is sufficiently secured.
When applying silver paste covering the side (edge) part of the anode substrate on the surface of which an oxide film layer, an electroconductive polymer layer and a carbon paste layer are sequentially laminated by a dipping method, i.e., dipping the anode substrate on which the above-mentioned layers are sequentially laminated into silver paste and then pulling it up, the silver paste layer deposited on the side (edge) part becomes thinner compared to that formed on the face part. As a result, the electric resistance to the electric current flowing from the bottom surface to the top surface becomes higher compared to the resistance to the current flowing alongside a surface of the anode substrate. Consequently, a solid electrolytic capacitor comprising silver paste applied by a dipping method tends to have higher ESR compared to a solid electrolytic capacitor comprising silver paste applied by a brush.
Meanwhile, when silver paste is applied by a brush, uneven application occurs and the thickness of a silver paste layer formed on the applied surface varies. Particularly, when a silver paste containing fluorine resin is used, the thickness tends to be uneven. When increasing the amount of silver paste to be applied so as to prevent thin portions of a silver layer, it generates unnecessarily thick portions of a silver layer, and thereby reduces the number of elements which can be encapsulated in a chip with a certain height and lowers the capacitance per unit volume of a capacitor.
Therefore, with respect to a solid electrolytic capacitor comprising an oxide film layer, an electroconductive polymer layer, a carbon paste layer and a silver paste layer sequentially formed on the surface of an anode substrate comprising valve-acting metal, a solid electrolytic capacitor is demanded which has a laminated structure of a silver paste layer on an anode substrate having a carbon paste layer on the surface and has a sufficiently low equivalent series resistance (ESR) and a high capacitance per unit volume.